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If I block a port in the Windows Firewall, and then attempt a connection to that port from another PC, what should I expect to see in Wireshark?

When running wireshark at the destination machine ('server' for lack of a better term), should I see:

  1. No inbound or outbound connections (which is what I would expect with a hardware firewall)
  2. An inbound connection attempt, but no outbound

At the moment I'm seeing #2 - I can see the inbound connection attempt, but I never see any response from the machine.

Should this be the expected behaviour?

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up vote 1 down vote accepted

Where is Wireshark running, on the destination machine or on the source machine? If it's running on the source machine I would expect to see behavior as in # 2. You should see the outgoing connection from the source machine in Wireshark but no return traffic. I'm curious, does the Windows Firewall send a RST when it blocks incoming connections or does it silently drop the connection?

Come to think of it, I might expect to see behavior as in # 2 on the destination machine as well due to the fact that the incoming connection has to be initiated before Windows firewall can do it's job. It can't block a connection that hasn't made it to the destination machine and up to layer 3 yet.

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Thanks Joe, I've updated my question to be clearer – Mark Henderson Jan 28 '10 at 3:09
Also, by the looks of it, it silently drops the connection, because there's absolutally no traffic going back to the source IP, not even a Refused notification. – Mark Henderson Jan 28 '10 at 3:12
That makes sense based on my assumption in my second paragraph. Wireshark works from layer 2 and up, while I'm guessing that the Windows firewall works at layer 3 and up (which is probably why you can only create rules that operate at layers 3 and up and not at layer 2). So Wireshark sees the connection at layers 2 and 3 before the Windows firewall acts. Once the Windows firewall acts on the connection, it kills it (I don't know what method it uses) and so there's nothing coming back down the stack for Wireshark to see. – joeqwerty Jan 28 '10 at 3:16
@Farseeker: Thanks for accepting my answer. It's nice to have someone who's technical savvy I admire give me a thumbs up. – joeqwerty Jan 28 '10 at 4:31
Admire? Well there's a word I don't think I've heard in my direction before. Anyhoo your ServerFault rep is looking quite healthy :) I was surprised to see you on here to be honest, I was expecting a different batch of users! – Mark Henderson Jan 28 '10 at 9:14

I would think so. The firewall is software within the O/S. The packet still has to come into the machine, bubble up through the networking stack until it hits the firewall, at which point it's accepted and passed or denied and blocked. I would expect this behavior.

Now if the firewall existed between the Internet and your machine, you would see nothing in Wireshark if that external firewall blocked the packet(s).

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I suspected as much. I've never really relied on the Windows firewall for anything before, I usually use a hardware firewall – Mark Henderson Jan 28 '10 at 3:10

You should receive an inbound connection attempt,because the firewall blocked the packets, so the machine will have no response to the request.

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